Three-Dimensional Spirals of Atomic Layered MoS₂

Abstract

Atomically thin two-dimensional (2D) layered materials, including graphene, boron nitride, and transition metal dichalcogenides (TMDs), can exhibit novel phenomena distinct from their bulk counterparts and hold great promise for novel electronic and optoelectronic applications. Controlled growth of such 2D materials with different thickness, composition, and symmetry are of central importance to realize their potential. In particular, the ability to control the symmetry of TMD layers is highly desirable because breaking the inversion symmetry can lead to intriguing valley physics, nonlinear optical properties, and piezoelectric responses. Here we report the first chemical vapor deposition (CVD) growth of spirals of layered MoS2 with atomically thin helical periodicity, which exhibits a chiral structure and breaks the three-dimensional (3D) inversion symmetry explicitly. The spirals composed of tens of connected MoS2 layers with decreasing areas: each basal plane has a triangular shape and shrinks gradually to the summit when spiraling up. All the layers in the spiral assume an AA lattice stacking, which is in contrast to the centrosymmetric AB stacking in natural MoS2 crystals. We show that the noncentrosymmetric MoS2 spiral leads to a strong bulk second-order optical nonlinearity. In addition, we found that the growth of spirals involves a dislocation mechanism, which can be generally applicable to other 2D TMD materials.

Face the Edges: Catalytic Active Sites of Nanomaterials

“…As a proof of concept, ultrathin spiral nanosheets with overgrown edges of NiFe, CoFe and CoNi bimetallic hydroxides are achieved now, and they show promising electrochemical activities for oxygen evolution reaction. The growth mechanism and influencing parameters are easy to accessible, and taking the achievement of several other spiral layered structures57, 58 into account, the fabrication of more spiral nanosheets with overgrown edges can be foreseen in the future. Since structure determines properties, the properties of such novel structures might be quite interesting.…”

Processable 2D materials beyond graphene: MoS2liquid crystals and fibres

“…The as-synthesized crystalline material shows large crystals (more than 10 µm) with screw dislocations in the form of hexagons on the surface due to a non-centrosymmetric arrangement of adjacent layers (Figure 2a and b). 31 In comparison and as a control material that was also exfoliated, large natural single crystals of MoS 2 (Manchester Nanomaterials) show a metallic surface finish with a flake size in the order of a few centimeters with no dislocations ( Figure 2c and d). Exfoliation was achieved by employing an organolithium compound as a lithiation agent to expand the lattice and obtain an intercalated compound, subsequently exfoliating it into 2D sheets by an ultrasound-assisted hydration process.…”

Scanning Tunneling Microscopy of Atomic Scale Phonon Standing Waves in Quasi-freestanding WSe2 Monolayers

“…Our key experimental findings: correlation between interference signals and quasifreestanding geometry of islands, bias independent 2a 0 periodicity of fringes, produced by BZ edge, which we directly observe on Fourier images, combined with symmetry breaking effect, observation of two types of interference patterns, and 3a 0 vs. 7a 0 absolute ring diameters (a) do not have analogues among other known forms of nanoscale interference phenomena and (b) are naturally explained by phonon standing waves mechanism. We would also like to mention here that the alternative interference mechanism, electronic Friedel oscillations [20], as earlier STM studies [14,21,22] have shown, is usually not manifested in monolayers of transition metal dichalcogenide (TMD) semiconductors, probably due to its suppression caused by lack of inversion symmetry [23].…”